1. Field of the Invention
The present invention relates generally to communications networks, and to a radio capable of operating in the presence of interference or jamming on network channels.
2. Discussion of the Known Art
Cellular telephones and other commercial off-the-shelf wireless devices are used by enemy combatants to trigger improvised explosive devices (IEDs) near roads where friendly forces and civilians are expected to travel. For example, a cell phone may be wired to an IED so as to detonate the device in response to a wireless call signal initiated by an enemy nearby. To counter the threat effectively, it becomes necessary to jam each radio frequency (RF) channel the enemy could possibly use to transmit the triggering call signal to a given location, including channels normally used for communications among military and civilian personnel. But while such “friendly” jamming can work to prevent the enemy from triggering explosives by use of common wireless devices, it is likely to degrade communications severely among friendly forces over the channels being jammed.
U.S. Pat. No. 6,687,506 (Feb. 3, 2004) discloses apparatus for disabling radio (i.e., cellular) telephones inside public premises such as theaters, schools, restaurants and the like, by jamming the reception of call signals entering the premises. See also, U.S. Pat. No. 6,393,254 (May 21, 2002), U.S. Pat. No. 7,050,755 (May 23, 2006), and U.S. Pat. No. 6,112,052 (Aug. 29, 2000). The '052 patent describes a remote controlled device for jamming enemy radio signals wherein a number of the devices are concealed and deployed at different locations in enemy territory.
U.S. Pat. Application Pub. No. 2002/0012411 (Jan. 31, 2002) and Pub. No. 2003/0112905 (Jun. 19, 2003) disclose a global positioning system (GPS) receiver capable of functioning in the presence of interfering or jamming signals by the use of notch filters. Both documents also state at par. [0011] that the disclosed approach is preferable over one that uses “cancellation” wherein a replica of the jamming signal is generated and then subtracted from the received signal. Such a cancellation process is disclosed in an article by M. Zhodzishsky, et al., “In-band Interference Suppression for GPS/GLONASS”, available on the Internet at <http://storage.javad.com/downloads/papers/interference_suppression.pdf>. The article describes the synthesis or modeling of detected narrowband GPS interferers and their active cancellation in a receiver.
So-called antenna noise canceller devices are commercially available from Timewave Technology Inc., model # ANC-4, and MFJ Enterprises Inc., model #1026. Both devices make use of a separate “noise antenna” arranged to sense strong RF noise signals from nearby sources such as light dimmers, fluorescent lamps, motors, power lines, lightening bursts, and the like. Signals sensed by the noise antenna are correlated with all RF signals sensed by a main receiving antenna, and proper amplitude and phase parameters are determined so that only the RF noise signals are canceled from other desired RF signals before entering the front end of the receiver.
The mentioned noise canceller devices obtain an essentially pure sample of a nearby source of noise or interference, correlate the sample with a desired communications signal from a more distant source, and determine the correct parameters to enable noise cancellation. In military and other tactical operations, however, an enemy jammer transmitter is usually located far from a receiver attempting to detect friendly communications signals on the jammed channels. This makes it impossible to obtain a pure sample of the jamming signal waveform alone. Therefore, to enable cancellation under such conditions, a proper model of the jamming signal waveform must be synthesized locally at the receiver.
U.S. Pat. Application Pub. No. 2003/0054755 (Mar. 20, 2003) describes a Coded Orthogonal Frequency Division Multiplexing (COFDM) receiver with an “anti-jamming” (AJ) processor. The receiver detects multiple subcarriers of a received signal carrying data, wherein one or more of the subcarriers may be subject to unintentional interference from cordless telephones, Bluetooth® transmitters, or other devices authorized to transmit at radio frequencies that are at or near the frequencies of the subcarriers. Any interference on each subcarrier is assessed, and corresponding reliability metrics are assigned to the subcarriers. The received signal is demodulated using the reliability metrics to recover the data, according to the '755 publication. See also, U.S. Pat. Application Pub. No. 2003/0143943 (Jul. 31, 2003).
Sophisticated wireless military communication systems and jamming systems have been developed as disclosed in, e.g., R. A. Poisel, Modern Communications Jamming Principles and Techniques, Artech House (2004); and O. Holt, “Technology survey: Sampling of communications jammers”, J. Electronic Defense, pages 43 to 46 (February 2008), all relevant contents of which are incorporated by reference.
In tactical situations involving a dense deployment of multiple RF communication systems, interference among the various systems can severely limit their individual performance. Improving spectrum interoperability among the systems is paramount if adverse effects of spectrum congestion are to be avoided. One technique currently being developed for efficient spectrum usage is so-called Dynamic Spectrum Access (DSA) as described in A. N. Mody, et al., “Recent advances in cognitive communications”, IEEE Comm. Mag., vol. 45, no. 10, pp. 54 to 61 (October 2007), all relevant contents of which are incorporated by reference. Radios using this technique detect, track, and predict frequency bands occupied by a potential interferer in time, frequency, and space, and the radios shift their operating frequencies accordingly in order to avoid the interference.
It is common to communicate on RF channels, and methods of disrupting or jamming such communications are also generally well known. As far as is known, however, a technique that allows a user to communicate reliably on a RF band or channel while the channel is jammed by the user or another source, has not been disclosed. In particular, there is a need for a radio that can communicate with other radios on one or more channels of a network while the same channels are jammed by either friendly or hostile entities.
Moreover, current techniques for enabling communications in the presence of jamming require a significant reduction in transmission data rates due to the need for powerful low rate error correction codes, spreading increases (direct sequence or frequency hopping), and certain collision avoidance techniques. Accordingly, there is also a need for a radio that can provide full-rate anti-jam (FRAJ) wireless communications in the presence of jamming signals whether friendly or hostile, as well as nearby RF emitters that can act as “unintentional” jammers.